Electrochemical CO2 Reduction to Methane

ABSTRACT

Nation-modified electrodes for the CO 2  reduction reaction (CO 2 RR) to hydrocarbon products. Depending on the thickness of the Nation membrane and its admixture with other polymers, CO 2  reduction occurs principally at the electrode-polymer interface. A Nation overlayer of 15 μm on a Cu electrode enables an extraordinarily high yield of CH 4  production (88% Faradaic efficiency) at a low overpotential (540 mV). Other embodiments directed to admixtures of Nation and other polymers and/or cocatalysts, various metal substrates and electrolyte solutions which comprise an aprotic solvent in addition to a bicarbonate solution show impact on the Faradaic efficiency, yield and carbon-based products produced by the present invention.

RELATED APPLICATIONS

This application claims the benefit of priority of U.S. provisionalapplication Ser. No. 63/014,338 filed Apr. 23, 2020, the entire contentsof which application is incorporated herein.

FIELD OF THE INVENTION

This invention relates to the production of methane and othercarbon-based chemical products in electrochemical reactions involvingthe reduction of carbon dioxide. This invention is also directed topolymer coated metal substrates (electrodes) which find use in reducingcarbon dioxide/bicarbonate to hydrocarbons, organic acids and alcohols,among other carbon-based products.

BACKGROUND AND OVERVIEW OF THE INVENTION

The accelerated increase of CO₂ concentrations in the atmosphere due toanthropogenic activities is causing a host of economic and environmentalissues such as coastal flooding, increased catastrophic weather events,shifting agricultural productivities, and decreased biodiversity. Theglobal CO₂ concentration measured at the Mauna Loa Observatory in April2021 was 418 ppm.¹ In the 1960s, CO₂ levels increased approximately 0.6ppm per year, and this rate rose to approximate 2 ppm per year in thelast decade.² To combat rising global CO₂ levels in a world with aneconomy heavily dependent upon fossil fuels, chemical carbon mitigationaims to capture atmospheric CO₂ and convert it to value-added products.³Electrochemical reduction of CO₂ to synthetic fuels using renewableenergy sources is a promising approach to store energy into chemicalbonds for industrial applications⁴ and is a renewable and efficientmethod of reducing CO₂ to various products based on multiple electrontransfer mechanisms.^(5,6,7,8)

Electrochemical CO₂ reduction has been of interest for many decadesbecause it is a viable pathway to produce synthetic fuels in aqueouselectrolytes and at room temperatures. This method presents a promisingpath towards establishing a carbon-neutral cycle.^(9,10) However, thereare still major drawbacks that limit the commercialization of CO₂reduction catalysts. The main problems associated with electrochemicalCO₂ reduction are the high overpotentials required to reduce CO₂, poorproduct selectivity, and low Faradaic efficiencies due to the hydrogenevolution reaction (HER) that occurs at similar reduction potentials asCO₂.^(11,12) The high overpotentials and poor product selectivity aredue to the adsorption energies of key reaction intermediates.^(13,14,15)Therefore, novel electrocatalysts for CO₂ reduction need to be designedthat are robust and selective while lowering overpotentials.

Of all the catalysts tested for electrochemical CO₂ reduction, Cu-basedmaterials are the only class of catalysts that have demonstrated highactivity toward more reduced hydrocarbons and alcohols.^(10,11,12,16,17)In 1985, electrochemical CO₂ reduction on metal electrodes was pioneeredby Hori and colleagues. Hori's work found that electrochemical CO₂reduction on a Cu electrode produced hydrocarbons, mainly methane (CH₄)and ethylene (C₂H₄).^(18,19,20) Jaramillo and coworkers found that Cuelectrodes produced 16 different products, out of which 12 are C2 or C3species.²¹ In an attempt to understand product selectivity and toelucidate the mechanism of CO₂ reduction, it was found that CO is a keyintermediate in the formation of CH₄ and C₂H₄,²² and that the productsof CO₂ reduction reaction depend on the metal's binding energy to CO.²¹Based on these findings, one strategy for efficient electrochemical CO₂conversion is to separate the process into two steps: CO₂ reduction toCO, followed by CO reduction to oxygenates and hydrocarbons.²³

Nafion is a sulfonated fluoropolymer which has been used in protonexchange membrane fuel cells (PEMFCs) and electrochemical CO₂ reductionreactions to separate the working electrode from the counter electrodeto prevent the re-oxidation of products. In a previous study by Kim andcoworkers, a thin layer of Nafion overlayer was introduced ontoPd-deposited TiO₂ nanoparticles, which enhanced the photo-conversion ofCO₂ to methane and ethane under UV and solar irradiation without the useof electron donor.²⁵

SUMMARY OF THE INVENTION

The present invention is directed to CO₂ reduction on polymeric,Nafion-modified electrodes and contemplates a mechanism in which CO₂reduction occurs in the presence of Nafion and Nafion based polymers.Previous work has only mixed catalysts with Nafion²⁶ or used Nafion toseparate the two sides of electrochemical devices.²⁷ The presentinvention steps beyond the prior art in controlling proton transport bythe thickness and composition of the Nafion layer on top of anelectrode, that is, on an electrode surface in contact with an effectivesolution, preferably, an aqueous biocarbonate solution, which mayinclude an aprotic reductively stable solvent such as acetonitrile(MeCN), dimethylformamide (DMF), dimethylacetamide (DMA),dimethylsulfoxide (DMSO), tetrahydrofuran (THF), propylene carbonate(PC) and alkyl nitriles (such as propylnitrile, butyl nitrile,adiponitrile, benzonitrile), among others.

In an embodiment, with an optimal Nafion layer, a copper electrodeproduces a remarkably high yield of methane (CH₄), (Faradaic efficiencyof 88.0%) at −0.38 V vs. RHE (reversible hydrogen electrode), which isevidently the highest yield for CH₄ production from a CO₂ reductionelectrocatalyst and an unexpected result. It is hypothesized that theNafion increases the CH₄ yield by stabilizing an intermediate in whichCO* is bound to the electrode surface and allows reduction of the COintermediate to methane. Additional experiments show that providing theNafion in admixture with at least one additional polymer at varyingweight percentages, such as polyvinylidene fluoride (PVDF),polyvinylpyrrolidine (PVP), polyethyleneglycol (PEG), polyvinylalcohol(PVA), polyethyleneimine (PEI), polytetrafluoroethylene (PTFE) andmixtures thereof, especially PVDF and PTFE, among others, enhances theproduction of alternative carbon products from CO₂ (bicarbonatesolution) such as formic acid (HCOOH), ethanol, ethylene, propylene and1-propanol, among others. Often, when a polymer is admixed with Nafion,the polymer has a CO₂ gas permeability ranging from 5×10⁻¹⁵mol-cm/cm²-s-Pa to 5×10⁻¹⁸ mol-cm/cm²-s-Pa. Among these polymers are thehighly permeable fluoropolymers polyvinylidene fluoride (PVDF) andpolytetrafluoroethylene (PTFE), which are characterized as having CO₂permeabilities of 2.16×10⁻¹⁷ mol-cm/cm²-s-Pa and 5.15×10⁻¹⁶mol-cm/cm²-s-Pa, respectively. Nafion has a CO₂ permeability of8.70×10⁻¹⁶ mol-cm/cm²-s-Pa. See, Flaconneche, et al., Oil Gas Sci.Technol.-Rev. IFP, 2001, 56(3), 261-278; Ren, et al., J. Electrochem.Soc., 2015, 162(10), F1221-F1230; and Giacobbe, et al., Matt. Lett.,1990, 9(4), 142-146. In still other embodiments, the polymer is admixedwith nanoparticles or nanowires of cocatalysts such as copper(metallic), cuprous oxide (Cu₂O), cupric oxide (CuO), Zn, zinc oxide(ZnO) or silver (Ag) or other metals to influence the Faradaicefficiency and/or the product mixture obtained from practicing thepresent invention.

In an embodiment, the invention is directed to metal substrateelectrodes which are uniformly coated with polymeric materialscomprising of Nafion polymer, alone or in admixture with other polymersand/or cocatalysts as described herein, which facilitates the efficientreduction of carbon dioxide into reduced carbon-containing chemicalcompounds including hydrocarbons (e.g., methane, ethane, propane,ethylene and/or propylene), organic acids and alcohols such as methanol,ethanol and 1-propanol, among others. Polymers (principally asdispersions of Nafion or Nafion and another polymer as described hereinranging from 1% to 20-25% by weight polymer, often about 5-15% by weightpolymer in aqueous solvent) are deposited onto metal substrates atuniform thicknesses ranging from 1 μm to 90-100 μm. Often the polymercoating has a uniform thickness of 1-30 μm, more often 1-20 μm or 2-15μm (for Nafion polymers) and 20 to 90-100 μm, often 20-90 μm (forNafion/other polymer admixtures) using methods which are well known inthe art, such as drop-casting, spin coating, spray-coating andblade-containing, among others known in the art. After deposition, thepolymer coating is dried (e.g. air-dried or dried using hot air dryer)to remove aqueous solvent and what remains is a uniform coating ofdesired thickness.

The polymer composition of the coating is often solely or principallyNafion (to produce methane gas efficiently, but the Nafion may beadmixed with another polymer such as polyvinylidene fluoride (PVDF),polytetrafluoroethylene (PTFE), polyvinylpyrrolidine (PVP),polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine(PEI) or mixtures thereof, especially PVDF and PTFE). In polymerovercoatings, the Nafion comprises between 5% and 100% by weight of thepolymer coating, often more than 40-50% by weight of the polymercoating, with the remaining portion of the polymer coating comprisingone or more of the above described polymers and/or cocatalysts inadmixture with the Nafion. In embodiments, a cocatalyst such asnanoparticles ranging from 1-500 nm in diameter or nanowires of copper(metallic), cuprous oxide (Cu₂O), cupric oxide (CuO), Zn, zinc oxide(ZnO) or silver (Ag) or other metals is added to the Nafion polymer orNafion polymer admixture in an effective amount, preferably ranging from0.5% to 50%, often 5% to 30% often 10-15%, most often approximately 10%by weight of the polymer coating. The inclusion of cocatalyst may assistin facilitating (increasing the Faradaic efficiency) the production ofand/or influencing the type of carbon products produced by the CO₂reduction reaction produced by the present invention. The cocatalystsare incorporated into the polymer coating by mixing the nanoparticleswith the polymer(s) to provide a uniform suspension by stirring,sonication and/or heating and the suspension of polymer and cocatalystnanoparticles and/or nanowires are deposited on the metal substrate bydrop-casting, spin coating, spray-coating and blade-containing, followedby drying to a uniform coating.

In an embodiment, the invention is directed to metal substrates(electrodes) which are coated with a uniform polymer coating and whichfunction as electrodes in a CO₂ reduction apparatus or cell as depictedin FIG. 1 hereof for electrochemically converting CO₂ tocarbon-containing chemical compounds pursuant to the methods which aredescribed herein. In embodiments, the metal substrate, which can vary insize and thickness over a wide range from a thin foil to a substrate ofsubstantial thickness, comprises carbon or a transition metal or atransition metal alloy or an intermetallic (i.e., an admixture of two ormore metals, at least one of which is a transition metal). Transitionmetals include metals which are found in the d-block of the periodictable, which includes groups 3-12 and periods 4-7 of the periodic table.These atoms have between 0 and 10 d-electrons. In embodiments, the metalsubstrate comprises a late transition metal of groups 8-12 of theperiodic table or an alloy thereof. In embodiments, the substratecomprises carbon or a late transition metal of groups 10-12, oftencopper, zinc, silver, gold, cadmium, nickel, palladium, platinum or analloy or intermetallic thereof, more often copper, nickel or zinc or analloy or intermetallic thereof. In embodiments, the metal substrate mostoften comprises copper, or a copper alloy or intermetallic, often brass(copper and zinc), bronze/phosphor bronze (copper and tin), naval brass(copper, zinc and tin), aluminum bronze (copper and aluminum),berylliumcopper (copper and beryllium), cupronickel (copper and nickel,optionally iron and/or manganese), nickel silver (copper with nickel andzinc), copper silver (copper with silver) and copper gold (copper withgold). All of the above-described transition metals or alloys are usefulas electrodes for conducting CO₂ reduction reactions.

In embodiments, the substrate/electrode may be any size or thicknessthat is appropriate for the apparatus or cell, including experimentalcells of relatively small size and commercial embodiments of great sizefor industrial applications. The size and thickness of the substratedoes not impact the rate (current density) or extent of product and isotherwise not a critical feature for the process of the presentinvention and the electrochemical reaction to reduce CO₂ produces thesame result because the reaction takes place on the electrode at thepolymer-electrode interface. The current of the reaction scales linearlywith the electrode area, so the reaction can work with any sizesubstrate.

In embodiments, the electrolyte solution is a bicarbonate solutionranging from 0.01 M to 1.1 M bicarbonate (the solubility of bicarbonatein water at room temperature), although solutions of 0.05 M to 0.2 M areoften used and 0.1M bicarbonate is most often used. In embodiments, anaprotic solvent is added to the electrolyte solution (at a volumepercent ranging from 1% to 95% of the electrolyte solution, often 20-80%by volume or 40-60% by volume and most often approximately 50% by volumeof the electrolyte solution to influence the organic products producedfrom the CO₂ reduction reaction. It was determined experimentally thatthe inclusion of an effective amount of an aprotic solvent such asacetonitrile (MeCN), dimethylformamide (DMF), dimethylacetamide (DMA),dimethylsulfoxide (DMSO), tetrahydrofuran (THF), propylene carbonate(PC) and alkyl nitriles (such as propylnitrile, butyl nitrile,adiponitrile, benzonitrile), among others tends to inhibit the reductionreaction to some extent (because fewer protons are available) resultingin products such as ethylene, methanol, ethanol, propanol and formicacid as well as carbon products of higher carbon number because of thepromotion of CO* intermediate dimerization or trimerization at theelectrode surface and the reduced proton concentration of the reductionenvironment.

In embodiments, to provide electrolyte solutions, CO₂ is often bubbledthrough a solution which may be buffered to maintain high localconcentrations of bicarbonate within the ranges specified above. The pHof the electrolyte solution generally reflects the concentration of thebicarbonate in solution with solvent and/or buffer effects influencingthe pH of the solution. At equilibrium solution concentration, the pH ofthe solution is approximately 6.8, although the pH may rangesubstantially depending on the concentration of the biocarbonate andother components (other solvents/buffering agents) in solution.

In embodiments, the metal substrate/electrode comprises a uniformpolymer layer on the surface of the substrate having a thickness rangingfrom 1 μm to 90-100 μm, with a polymer which contains Nafion as its solepolymeric component ranging from 1 μm to 30 μm, often 2 μm to 20 μm or 2μm to 15 μm. In the case of admixtures of Nafion and other polymers,often fluoropolymers such as polyvinylidene fluoride (PVDF) and/orpolytetrafluoroethylene (PTFE) or other polymers such aspolyethyleneglycol (PEG), polyvinylalcohol (PVA) or polyethyleneimine(PEI) as described herein, the thickness of the coating on the metalsubstrate will often range from 20-100 μm and above, often 20-90 μm.

Generally, the CO₂ reduction reactions of the present invention areconducted within the apparatus or cell using a voltage ranging from −0.2V to −2 V vs. RHE (reversible hydrogen electrode). The current(expressed as current density) which is used in the electrolyticprocesses to reduce CO₂ to carbon-based products as described hereinranges from 1-100 milliamps per cm², often 10-100 milliamps per cm².

In embodiments, a high amount of methane gas (CH₄) is produced using auniform Nafion polymer (alone) overcoating ranging from 2 to 15 μm on acopper electrode (Faradaic efficiency of 50+%) at an effective voltage(very negative reduction potentials). In embodiments, methane gas (CH₄)is produced using a uniform Nafion polymer (alone) overcoating ofapproximately 15 μm on a copper electrode (Faradaic efficiency of 88.0%)at −0.38 V vs. RHE (reversible hydrogen electrode).

In embodiments, the inclusion of effective amounts of an additionalpolymer in admixture with Nafion (in embodiments, the polymer ispolyvinylidene fluoride (PVDF), polyvinylpyrrolidine (PVP),polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine(PEI), polytetrafluoroethylene (PTFE) or mixtures thereof) favors theproduction of formate at less negative reduction potentials.

In embodiments, the production of ethylene gas is favored when copperalloys are used, when the alloy electrode has a hydrophobic coatingcomprising an effective amount of PVDF in admixture with Nafion and whenaprotic solvents as otherwise described herein (often acetonitrile) areused in effective amounts in combination with a bicarbonate in theelectrolyte solution. Thus, the invention provides that methane gasformulation is favored using Nafion copolymer (in the absence of anyother copolymer) of uniform thickness between 2 and 15 μm or 10 and 15μm, more often approximately 15 μm at an effective voltage between −0.2V and −2.0 V vs. RHE. In embodiments, the production of formate isfavored in a hydrophobic polymer environment comprising a uniformoverlayer of Nafion in combination with an effective amount ofcopolymer, especially PVDF, as described herein above. In embodiments,ethylene production is favored by the use of hydrophobic fluoropolymer(PVFD and/or PTFE) in admixture with Nafion on an alloy (often copperalloy) electrode. In embodiments, the inclusion of a nanoparticulate,nanowire cocatalyst or covalently bonded cocatalyst into the Nafionpolymer or additional polymer may enhance the formation of CO*intermediates and methane and/or ethylene products, especially on copperelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a CO₂ reduction apparatus having a threeelectrode configuration for carrying out reduction of CO₂ to variouscarbon-based products pursuant to the present invention as otherwisedisclosed herein. As illustrated in FIG. 1 , an electrochemicalapparatus or cell for the production of a gas such as methane fromcarbon dioxide comprises a body member or housing 10 that defines achamber 12. A reference electrode 30 extends from a cap or cover member16 through an insulating seal 18 axially down into the chamber 12. Adistal end portion 20 of reference electrode 30 is disposed in a cavityor chamber extension 22 at the bottom of chamber 12. A working electrode24 as described in detail herein is disposed at a lower end of cavity22, sandwiched between a shoulder (not designated) of housing 10 and abase plate 28. A counter electrode 14, co-functioning with workingelectrode 24 extends into chamber 12 from cover member 16 and throughinsulator-seal 18. Electrically conductive structures 32 and 34 areprovided in cover member 16 for operatively connecting referenceelectrode 30 and counter electrode 14 to a voltage source 36. Workingelectrode 24 is connected to voltage source 36 via a copper foil 42disposed adjacent working electrode 24 for electrical conduction. Twoport members or fittings 38 and 40 are fixed to housing 10 on oppositesides thereof and communicate with chamber 12. Carbon dioxide gas is fedinto chamber 12 via port member or fitting 38, while gas containingelectrochemical product such as methane is conveyed out of the cellhousing 10 via port member or fitting 40. Working electrode 24 is acathode for purposes of the voltage of −0.2 to −2 V. Counter electrode14 serves as an anode. Direct current is principally used, referenceelectrode 30 serving to maintain a constant voltage between −0.2 and −2volts. Alternatively, oscillating current (AC) could be applied.

The apparatus shown in FIG. 1 is presented as a three-electrodeconfiguration comprising a working electrode (where reduction of CO₂ tocarbon-based products pursuant to the present invention takes place), areference electrode (which is used to maintain a constant voltageapplied to the working electrode) and a counter electrode (which is usedto as the counter electrode to the working electrode—in preferredaspects of the present invention as an anode counter to the workingelectrode, which is a cathode). In embodiments, the apparatus is atwo-electrode configured cell with the reference electrode beingeliminated from the apparatus.

FIG. 2A is a graph, specifically a linear sweep voltammogram of carbonwithout Nafion (top curve), with a 2 μm Nafion overlayer (middle curve,right side of figure), and with a 15 μm Nafion overlayer (lower curve,right side of figure), each in CO₂-saturated 0.1 M NaHCO₃ electrolytecarbon in CO₂-saturated 0.1 M NaHCO₃ electrolyte at a scan rate of 10mV/s.

FIG. 2B is a graph, specifically a linear sweep voltammogram of copperfoil without Nafion (top curve), with a 2 μm Nafion overlayer (middlecurve, right side of figure), and with a 15 μm Nafion overlayer (lowercurve, right side of figure), each in CO₂-saturated 0.1 M NaHCO₃electrolyte carbon in CO₂-saturated 0.1 M NaHCO₃ electrolyte at a scanrate of 10 mV/s.

FIG. 3A is a graph of electrochemical impedance spectroscopy (EIS) ofNafion-modified carbon taken using a three-electrode configuration at−0.89 V vs. RHE in 0.1 M NaHCO₃ electrolyte saturated with CO₂.

FIG. 3B is a graph of electrochemical impedance spectroscopy (EIS) ofNafion-modified copper taken using a three-electrode configuration at−0.89 V vs. RHE in 0.1 M NaHCO₃ electrolyte saturated with CO₂.

FIG. 4A shows graphs, specifically linear sweep voltammograms of carbonand Cu foil (next to lowest and uppermost curves at extreme left offigure) and carbon and Cu foil modified with 15 μm of PVDF (next tohighest and lowest curves at extreme left of figure) in CO₂-saturated0.1 M NaHCO₃ electrolyte at a scan rate of 10 mV/s.

FIG. 4B is a pair of electrochemical impedance spectroscopy (EIS) plotsof 15 μm of PVDF respectively on carbon and Cu, taken at −0.89 V vs.RHE.

FIG. 5 is a graph, showing linear sweep voltammograms of a carbon meshelectrode in 0.1 M NaHCO₃ saturated with CO₂ (lower line on left side)and 0.1 M NaHCO₃ adjusted to pH of 2.6 with HCl and saturated with CO₂(upper line on left side) at a scan rate of 10 mV/s.

FIGS. 6A and 6C-6F are images, while FIG. 6B is a graph, showing surfacecharacteristics of Nafion-modified electrodes. FIG. 6A is a scanningelectrode microscopy (SEM) image, while FIG. 6B is an EDS spectrum.FIGS. 6C-F show EDS mapping of a Cu electrode modified with a 2 μm thicklayer of Nafion. FIGS. 6D, 6E and 6F shows elemental mapping of the Cuelectrode for fluorine (FIG. 6D), oxygen (FIG. 6E) and sulfur (FIG. 6F).

FIG. 7 is a cross-sectional scanning electron microscope (SEM) image ofa 2 μm thick layer of Nafion on Cu foil.

FIGS. 8A, 8C, and 8D are images, w % bile FIG. 8B is a graph, alsoshowing surface characteristics of Nafion-modified electrodes. FIG. 8Ais a SEM image. FIG. 8B shows an EDS spectrum. FIGS. 8C and 8D show EDSmapping of Cu electrode modified with 8 μm of Nafion.

FIGS. 9A, 9B, and 9C are diagrams depicting three possibilities of CO₂reduction occurring at a polymer-electrolyte interface (FIG. 9A), apolymer-electrode interface (FIG. 9B), or an electrode-electrolyteinterface (FIG. 9C).

FIG. 10A is a graph showing Faradaic efficiencies for formate, CH₄, andCO for all catalysts at −0.89 V, while FIG. 10B is a graph showingpartial charge densities, and FIG. 10C is a graph showing rates ofproduct formation on bare substrates, 15 μm Nafion-modified substrates,and 15 μm PVDF-modified substrates.

FIG. 11A is a graph showing Faradaic efficiencies as a function ofNafion thickness on Cu foil substrate at −0.89 V vs. RHE, while FIG. 11Bis a graph showing partial charge densities, and FIG. 11C is a graphshowing rates of product formation as a function of Nafion thickness onsubstrate.

FIG. 12A is a graph showing Faradaic efficiencies of CO, CH₄, and HCOOHas a function of voltage for a 15 μm thick Nafion overlayer on Cu foil.FIG. 12A is a graph also showing Faradaic efficiencies of CO, CH₄, andHCOOH as a function of partial current density for a 15 μm thick Nafionoverlayer on Cu foil. FIG. 12A is a graph also showing Faradaicefficiencies of CO, CH₄, and HCOOH as a function of rates of productformation for a 15 μm thick Nafion overlayer on Cu foil.

FIG. 13 is a diagram showing a proposed mechanism of CO₂ reduction to COand CH₄. CO₂ adsorbs onto the electrode surface, with the addition of 2H⁺ and 2 e⁻ is reduced to a CO intermediate with two possible resonancestructures (shown in dotted box). Both structures are capable of eitherbeing released as gaseous CO or further reduction to CH₄.

FIG. 14 is a diagram showing proposed mechanism of CO₂ reduction to CH₄using a polymer-modified Cu electrode. CO₂ is reduced to CO at thepolymer-electrode interface. CO that is not bound to the electrodesurface is released as a product, and CO that is bound to the electrodesurface is denoted as a ═C═O* intermediate. Nafion helps stabilize thisintermediate allowing for the subsequent reduction to CH₄ whilepreventing CO release.

FIGS. 15A and B are cross-sectional SEM images of a Cu electrodemodified with a PVDF-Nafion polymer overlayer. FIG. 15A shows a 100 μmthick polymer layer and FIG. 15 B shows a 20 μm thick polymer layer.

FIG. 16A shows a cross-sectional SEM image of a Cu electrode modifiedwith a PVDF-Nafion polymer overlayer. FIG. B-D show EDS elementalmapping of F (FIG. 16B), O (FIG. 16C) and Cu (D) of the same Cuelectrode modified with a PVDF-Nafion polymer overlayer.

FIG. 17A shows photographic image of the contact angle of a waterdroplet on a bare Cu electrode. FIGS. 17B-D show photographic images ofthe contact angle of a water droplet on a Cu electrode modified withNafion-PVDF overlayers containing 30 wt. % PVDF (FIG. 17B), 52 wt. %PVDF (FIG. 17C), and 100 wt. % PVDF (FIG. 17D).

FIG. 18 shows linear sweep voltammograms (LSV) of bare Cu (black), Cumodified with 15 μm Nafion (red), Cu modified with 52, 60, and 100 wt. %PVDF in Nafion overlayer (blue, green, and purple) in CO₂-saturated 0.1M NaHCO₃ electrolyte at a scan rate of 10 mV/s.

FIG. 19A-C shows the Faradaic efficiencies (FIG. 19A), the partialcharge density over the 1 hour experiment (FIG. 19B), and rate offormation (FIG. 19C) for formate, CO, and CH₄ produced from 20-90 μmPVDF-Nafion-modified Cu at −0.89 V vs. RHE. The PVDF-Nafion overlayerbecomes increasingly thick as the weight percentage of PVDF increases.

FIG. 20A-C shows the Faradaic efficiencies (FIG. 20A), the partialcharge density over the 1 hour experiment (FIG. 20B), and rate offormation (FIG. 20C) for formate, CO, and CH₄ produced from 52 wt. %PVDF in Nafion modified Cu at different voltages.

FIG. 21A-C shows the Faradaic efficiencies (FIG. 21A), the partialcharge density over the 1 hour experiment (FIG. 21B), and the rate offormation (FIG. 21C) for gas products produced from unmodified Cu inacetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 22A-C shows the Faradaic efficiencies (FIG. 21A), the partialcharge density over the 1 hour experiment (FIG. 21B), and rate offormation (FIG. 21C) for liquid products produced from unmodified Cu inacetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 23A-C shows the Faradaic efficiencies (FIG. 23A), the partialcharge density over the 1 hour experiment (FIG. 23B), and rate offormation (FIG. 23C) for gas products produced from Cu modified with 15μm Nafion overlayer in acetonitrile/bicarbonate electrolyte at −0.89 Vvs. RHE.

FIG. 24A-C shows the Faradaic efficiencies (FIG. 24A), the partialcharge density over the 1 hour experiment (FIG. 24B), and rate offormation (FIG. 24C) for liquid products produced from Cu modified with15 μm Nafion overlayer in acetonitrile/bicarbonate electrolyte at −0.89V vs. RHE.

FIG. 25 shows the total carbon-containing products produced from anunmodified Cu electrode (FIG. 25A) and a Cu electrode modified with 15μm Nafion overlayer (FIG. 25B) in varying concentrations of acetonitrilein the bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 26 shows a proposed mechanism of high formate production using Cumodified by 52 wt. % PVDF in Nafion polymer overlayer (FIG. 26A). Theblue curved lines represent Nafion and the grey spheres represent PVDF.In this hydrophobic environment, formate is the preferred productbecause formate is the only CO₂ reduction product that does not generatewater, and generating water is unfavorable in a hydrophobic environment.Proposed mechanism of ethylene formation on a Cu electrode in anacetonitrile/bicarbonate electrolyte (FIG. 26B). Adding an aproticsolvent decreases the total proton concentration, which subsequentlydecreases the rate of M-CO protonation. This aprotic environmentpromotes M-CO and M-CO coupling to generate C2+ products instead ofprotonating M-CO to generate CH₄.

FIG. 27 shows a proposed mechanism of CO₂ reduction to C₂H₄ using aNafion-modified Cu electrode. The black dots embedded in the Nafionrepresents a hydrophobic polymer that slows proton transfer. CO₂ isreduced to CO at the polymer-electrode interface, and without rapidproton transfer to protonate the CO* intermediate, the stabilized CO*intermediates are allowed to dimerize which eventually produces C₂H₄.Trimerization (mechanism not shown) to selectively produce C3 productsis also hypothesized at even slower proton transfer rates.

FIG. 28 shows the CO Faradaic efficiency as a function of Nafionthickness on brass foil substrate at −0.89 V vs. RHE over 1 hourexperiment. Brass composition: 62% Cu, 37% Zn, trace amounts of Fe(<0.15%), Pb (<0.08%), and Sn (<0.005%).

FIG. 29 shows the Faradaic efficiencies of CO and CH₄ as a function ofvoltage on brass foil over a 1 hour experiment.

FIG. 30 shows the Faradaic efficiencies over a 1 hour experiment for COand C₂H₄ produced from 20-90 μm PVDF-Nafion-modified brass at −0.89 Vvs. RHE. The PVDF-Nafion overlayer becomes increasingly thick as theweight % PVDF increases.

FIG. 31 shows Faradaic efficiencies over a 1 hour experiment for CO andC₂H₄ produced from unmodified brass foil in acetonitrile/bicarbonateelectrolyte at −0.89 V vs. RHE.

FIG. 32 shows Faradaic efficiencies over a 1 hour experiment for CO,CH₄, and C₂H₄ produced from brass foil modified with 15 μm Nafion inacetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 33 shows Faradaic efficiencies over the 1 hour experiment for CO,CH₄, HCOOH, and CH₃OH produced from Zn foil modified with 15 μm Nafionin acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 34 shows CO Faradaic efficiency as a function of Nafion thicknesson Zn foil substrate at −0.89 V vs. RHE over 1 hour experiment.

FIG. 35 shows Faradaic efficiencies over the 1 hour experiment of COproduced from 20-90 μm PVDF-Nafion-modified Zn at −0.89 V vs. RHE. ThePVDF-Nafion overlayer becomes increasingly thick as the weight % PVDFincreases.

FIG. 36 shows Faradaic efficiencies over a 1 hour experiment for CO andC₂H₄ produced from 52 wt. % of polyvinylpyrrolidone (PVP), polyethyleneglycol (PEG), polyvinyl alcohol (PVA) and polyethylenimine (PEI) inNafion on a Cu substrate at −0.89 V vs. RHE.

FIG. 37 shows Faradaic efficiencies over a 1 hour experiment for CO andC₂H₄ produced from various polymer blends in Nafion on a Cu substrate at−0.89 V vs. RHE. (A=100 wt. % Teflon on Cu, B=50 wt. % each of Teflonand PVDF on Cu, C=52 wt. % Teflon in Nafion on Cu, D=40 wt. % each ofTeflon and PVDF in Nafion on Cu, E=64 wt. % Teflon and 30 wt. % PVDF inNafion on Cu, F=30 wt. % Teflon and 64 wt. % PVDF in Nafion on Cu.).

FIG. 38 shows Faradaic efficiencies over the 1 hour experiment for CO,CH₄, C₂H₄, and HCOOH produced from nanoparticulate Cu₂O on various metalsubstrates at −0.89 V vs. RHE. (A=10 wt. % Cu₂O dispersed in Nafion onCu, B=10 wt. % Cu₂O dispersed in Nafion on Zn, C, D, E=Cu₂O thin film onZn. Cu, and Ni metal substrates, respectively.)

FIG. 39 shows electrocatalysis at a polymer electrode interface with anembedded cocatalyst.

DETAILED DESCRIPTION OF THE INVENTION

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the,” include plural referents unlessexpressly and unequivocally limited to one referent. Thus, for example,reference to “a compound” can include two or more different compoundsdepending on the context of the use of the term. As used herein, theterm “include” and its grammatical variants are intended to benon-limiting, such that recitation of items in a list is not to theexclusion of other like items that can be substituted or other itemsthat can be added to the listed items.

The term “effective” is used to describe an amount of a component, anelement, an energy source, a reactant, precursor or product which isused in or produced by the present invention to produce an intendedresult.

The term “uniform” is used to describe the polymer overcoating which isused to coat the metal substrate pursuant to the present invention. Asused, a uniform overcoating is a coating on a metal substrate pursuantto the present invention which has a measured thickness at all areas ofthe coating within 10%± of the designated thickness.

The term “Faradaic efficiency” (synonymously faradaic yield, coulombicefficiency or current efficiency) is used to describe the efficiencywith which charge (electrons) is transferred in a system facilitating anelectrochemical reaction, in the present invention, the reduction of CO₂to one or more carbon-containing products. In other words, Faradaicefficiency is the percent yield of product based on the number ofelectrons transferred during the reaction. A higher percentage yield ofproduct using a lower number of transferred electrons provides higherFaradaic efficiency. In the present invention, the number of electronsis the limiting reactant, not carbon dioxide and a higher Faradaicefficiency is the desired outcome. Many CO₂ reduction catalysts have lowFaradaic efficiencies for carbon products in aqueous electrolytesbecause (fast) electron transfer can also occur to protons in water tocreate hydrogen gas, reducing the yield of the desired carbon product.The regulated proton transfer rates with the polymer overcoatings usingin the present invention very often increases the Faradaic efficiency ofthe CO₂ reduction reaction(s), a particularly favorable and unexpectedresult.

The inventors found that there was a relationship between the Faradaicefficiency of the CO₂ reduction reaction and the selectivity of thecarbon-based products which are produced reflective of the polymericovercoating and electrolyte solution used. For example, the inventorsfound that an extraordinarily high amount of CH₄ (at 88% Faradaicefficiency) is generated using a Cu electrode modified with a 15 μmNafion overlayer at −0.4 V vs. RHE. In contrast to the presentinvention, on unmodified metal electrodes, a more negative voltage isrequired to give rise to higher Faradaic efficiencies of CH₄. As shownin the examples section hereof, high formate Faradaic efficiencies canbe achieved by using PVDF-Nafion overlayers at a less negative voltage.Formate is favored in a hydrophobic environment because producing wateras a CO₂ reduction product is unfavorable. Since formate is the only CO₂reduction product in which water is not produced concomitantly, ahydrophobic electrode favors formate production pursuant to the presentinvention.

Thus, as shown in the examples section, a copper electrode modified with52 wt. % PVDF in Nafion at −0.14 V vs. RHE gives reasonably high formateyield (58%). This yield of formate is fairly high for a Cu-basedcatalyst, and most previous works used other metals to produce highformate yields such as 81% and 98%. There is some literature precedent,however, for Cu-based catalysts that achieve high formate yieldsincluding a Cu—Au catalyst that produces formate at a 81% Faradaicefficiency at −0.4 V vs. RHE.⁵ Cu₂O nanoparticle films also generatedformate at 98% Faradaic efficiency under high pressure (≥45 atm) at−0.64 V vs. RHE. The authors of this work also found that at morenegative potentials formate decreased.⁶ Comparing the present inventionto previous studies it seems that formate production is favored at lowervoltages, especially around from −0.4 to −0.6 V vs. RHE.

Reasonably high yields of C₂H₄ (75%) is generated when an alloysubstrate is modified by PVDF-Nafion overlayers on a Cu—Zn alloy (brass,62% Cu and 37% Zn). In addition, C₂H₄ is produced in the presence ofacetonitrile in the bicarbonate electrolyte (higher volume percent, ie.75% of acetonitrile generates more C₂H₄ than lower volume percent).Lastly, Cu electrodes modified with Teflon-Nafion overlayers favor theproduction C₂H₄ while simultaneous hindering CO production. Chen andcoworkers fabricated Cu, Cu—Ag, and Cu—Sn alloy films that exhibitedhigh Faradaic efficiencies (60%) for C₂H₄ production.⁷ The origin of thehigh C₂H₄ production is attributed to the presence of alloys, whichleads to the increased CO density on the electrode surface. In addition,higher local pH near the electrode surface also contributes to C₂H₄production because CO* dimerization and C₂H₄ formation.

In addition, alcohols such as methanol, ethanol, and 1-propanol aregenerated in the presence of acetonitrile/bicarbonate electrolyte onunmodified Cu electrodes or Cu electrodes modified with 15 μm Nafionoverlayer.

The term “Nafion” is used to describe Nafion (CAS NamePerfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid-tetrafluoroethylenecopolymer, also IUPAC name 1,1,2,2-tetrafluoroethene;1,1,2,2-tetrafluoro-2-[1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluoroethenoxy)propan-2-yl]oxyethanesulfonicacid), which is a sulfonated fluoropolymer which has a hydrophobicperfluorinated polytetrafluoroethylene (PTFE) backbone with side chainsterminated by strongly acidic hydrophilic sulfonic acid groups. Theprotons on the sulfonic acid groups are responsible for providing protonconductivity. Nafion can be formulated as a dispersion in water/alcohol(ethanol/I-propanol) in the acidic form. A preferred dispersion ofNafion, Nafion D520, D521, D2020 and D2021 (with Nafion polymer in thedispersion ranging from 5% by weight up to 20% by weight) can bepurchased from the Chemours Company, Wilmngton Del., USA. Nafion may beadmixed with other polymers to form admixtures which are used asovercoatings of the metal substrate in the present invention.

While not being limited by way of theory, it appears that Nafion hasprovided enhanced efficiency of CO₂ reduction in the present inventionfor at least the following three reasons, among others. First, Nafion isa gas permeable superacid and an excellent proton conductor, and it isbelieved that the Nafion layer enhances the local activity of protons onthe surface of the metal substrate which are necessary for increasedFaradaic efficiency of CO₂ reduction. Second, CO* is believed to bestabilized between the substrate/polymer interface, which would favorelectron transfer to the intermediates to form more highly reducedproducts, especially when considering the enhanced local activity ofprotons by Nafion. Third, Nafion is stable against photocatalyticoxidation and is inert toward photoinduced redox reactions, thus forcingthe equilibrium reactions toward reduction products rather than back tooxidized precursors.

The term “overpotential” is used to describe the difference in potentialwhich exists between a thermodynamically determined reduction potentialof a half-reaction and the potential at which the redox event isexperimentally observed. The term is directly related to a cell'svoltage efficiency. In an electrolytic cell the existence ofoverpotential indicates that the cell requires more energy than isthermodynamically expected to drive a reaction. The quantity ofoverpotential is specific to each cell design and varies across cellsand operational conditions, even for the same reaction. Overpotential isexperimentally determined by measuring the potential at which a givencurrent density is achieved.

Mechanism of Action

A following description of proposed mechanisms for CO₂ reductionpursuant to the present invention provides a basis for the formation ofmethane, formate, ethylene and other carbon-based produced according tothe present invention. FIGS. 14, 26 and 27 provide proposed mechanismsfor methane, formate and ethylene formation. The Nafion overcoatstabilizes the M-CO intermediate, which allows for subsequentprotonation to methane. Formate is produced when the electrode ishydrophobic (facilitated by higher concentrations of Teflon and/or PVDFin admixture with Nafion) because CO₂ reduction to formate does notrequire the production of water. Ethylene is favored as a carbon-basedproduct when the electrolyte solution (bicarbonate source) comprisessubstantial quantities of acetonitrile by volume. The rate determiningstep (RDS) of ethylene formation is the dimerization of the CO*intermediate.

FIG. 14 shows the proposed mechanism of CO₂ reduction to methane (CH₄)using a polymer modified copper electrode. As shown, CO₂ is reduced toCO at the electrode-polymer interface. CO that is not bound to theelectrode surface is released as a product. Nafion stabilizes thisintermediate allowing for the subsequent reduction to methane whilepreventing CO release from the surface of the electrode.

FIGS. 26A and 26B show the proposed mechanism of high formate productionusing Cu modified by 52 wt. % PVDF in Nafion polymer overlayer (FIG.26A). The blue curved lines represent Nafion and the grey spheresrepresent PVDF. In this hydrophobic environment, formate is thepreferred product because formate is the only CO₂ reduction product thatdoes not generate water, and generating water is unfavorable in ahydrophobic environment. Proposed mechanism of ethylene formation on aCu electrode in an acetonitrile/bicarbonate electrolyte (FIG. 26B).Adding an aprotic solvent decreases the total proton concentration,which subsequently decreases the rate of M-CO protonation and favoringthe production of higher carbon products, especially ethylene andalcohols such as methanol, ethanol and 1-propanol. This aproticenvironment promotes M-CO and M-CO coupling to favor the production ofC2+ products instead of protonating M-CO to generate CH₄.

FIG. 27 shows a proposed mechanism of CO₂ reduction to C₂H₄ using aNafion-modified Cu electrode. The black dots embedded in the Nafionrepresents a hydrophobic polymer that slows proton transfer. CO₂ isreduced to CO at the polymer-electrode interface, and without rapidproton transfer to protonate the CO* intermediate, the stabilized CO*intermediates are allowed to dimerize which eventually produces C₂H₄.Trimerization (mechanism not shown) to selectively produce C3 productsis also believed to occur at even slower proton transfer rates.

The above-described mechanisms are useful in predicting carbon-basedproducts that can be produced pursuant to features of the presentinvention. For example, methane (CH₄) production is favored whenelectrodes are modified with a Nafion overlayer and on unmodifiedelectrodes at a very negative reduction potentials. The Nafion overlayeror coating provides unexpectedly high Faradaic efficiency for theproduction of methane. Formate is favored with hydrophobic electrodes(PVDF-Nafion overlayer) and at less negative reduction potentials.Ethylene (C₂H₄) is favored when Cu alloys are used, when the alloyelectrode is hydrophobic (PVDF-Nafion overlayer), and when aproticsolvents are used in conjunction with the bicarbonate electrolyte. Theinventors have concluded that formate can be further enhanced bycreating a hydrophobic environment. C₂H₄ production can be enhanced byhydrophobic fluoropolymer-Nafion overlayers on an alloy electrode.

The following non-limiting examples further describe and supportembodiments and further aspects of the present invention.

EXAMPLES First Set of Experiments

The first set of experiments presented herein are directed to a study ofNafion-modified electrodes for the CO₂ reduction reaction (CO₂RR) tohydrocarbon products. Nafion, described herein above, is a sulfonatedpolymer possessing high proton conductivity. By varying the thickness,substrates, and voltage, the inventors performed a detailed study of theeffect of Nafion on metal and carbon mesh electrodes for CO₂ reduction.These studies allowed for the elucidation of the mechanism in which CO₂reduction occurs on these Nafion-modified electrodes. Depending on thethickness of the polymeric membrane surface, CO₂ reduction occurs ateither the polymer-electrolyte interface or electrode-polymer interface.It was determined that a Nafion overlayer of 15 μm on Cu electrodeenables extraordinary high yield of CH₄ production (88% Faradaicefficiency) at a low overpotential (540 mV). To the best of ourknowledge, this yield is the highest reported for electrocatalytic CO₂reduction to CH₄ production at room temperature reported thus far. Otherproducts detected include formate, CO, ethanol and methanol.

Experimental Procedure

Materials and Electrode Preparation.

Nafion D520 dispersion and carbon paper (AvCarb EP40T) were purchasedfrom Fuel Cell Store. Cu and Zn foil were purchased from All-Foils, Inc,and Ni foil was purchased from Goodfellow, Inc. Sodium bicarbonate waspurchased from Sigma Aldrich. CO₂ and CO were purchased from Airgas.Nafion-modified electrodes were fabricated by drop-casting Nafion (D520Dispersion) directly onto the substrate.

Electrochemical Measurements and Material Characterization.

All electrochemical measurements were performed using a VSP-300 BiologicPotentiostat. All electrochemical data were collected versus a Ag/AgClreference electrode and converted to the reversible hydrogen electrode(RHE) scale by V_((vs. RHE))=V_((measured vs. Ag/AgCb)+0.21+0.059*6.8(where 6.8 is the pH of solution). All values are reported versus RHE.To evaluate the CO₂ reduction activity of the thin films, the workingelectrodes were studied in 0.1 M sodium bicarbonate buffer sparged withCO₂ gas for at least 30 min using a one-compartment, three-electrodeconfiguration (as set forth in FIG. 1 hereof). The thin films on carbonpaper served as the working electrode, a Pt wire was used as the counterelectrode, and a Ag/AgCl electrode was used as the reference electrode.Electrochemical impedance spectroscopy (EIS) was performed in 0.1 Msodium bicarbonate buffer sparged with CO₂ gas using a three-electrodeconfiguration cell at −0.89 V vs. RHE. The frequency was varied from 200kHz to 100 mHz sinusoidally with amplitude of 10 mV. Scanning electronmicroscope (SEM) images and energy-dispersive X-ray (EDS) analysis wereobtained for each sample using a JEOL JSM-6010LA analytical SEM or aJEOL JSM-7100F field emission SEM operated using an accelerating voltageof 15 kV. Onset potentials were calculated by determining the voltage atwhich the current density reached 15% of the maximum current density foreach linear sweep voltammogram.

Product Determination. Electrochemical reactions were performedchronoamperometrically at −0.89 V vs. RHE (and at −0.38 V, −0.13 V, and0.12 V vs. RHE for voltage-dependent experiments) for one hour usingcarbon as a counter electrode in a beaker for determining liquid andsolid products and Pt wire as a counter electrode in a custom-made cellfor determining gas products. During chronoamperometry, CO₂ wascontinuously sparged through the solution at a rate of 5 cm³/min. Liquidproducts were quantified using a Varian 400 MHz NMR Spectrometer usingDMF as an internal standard. The water in the reaction solution wasevaporated under reduced pressure, and sodium formate along with otherresidual solids from the electrolyte were collected and dissolved inD₂O. Liquid products were extracted from the reaction solution usingdeuterated chloroform. Gas products were quantified using a SRI 8610Cgas chromatograph equipped with a flame ionization detector (FID) and amethanizer. The limits of detection for formate, liquid products, andgas products were determined to be 11 μM, 85 μM, and 1 ppm,respectively.

FIGS. 2A and 2B show linear sweep voltammograms of carbon mesh and Cusubstrates with and without Nafion overlayers in CO₂-saturatedbicarbonate electrolyte undergoing electrochemical CO₂ reduction.Unmodified carbon mesh (FIG. 2A, uppermost curve) reaches a maximumcurrent density of about −4 mA/cm² at −1.5 V vs. RHE and exhibits anonset potential of −0.29 V vs. RHE. In contrast, carbon mesh modifiedwith a 2 μm and 15 μm thick Nafion overlayer both exhibit a decreasedonset potential of −0.20 V and +0.25 V, respectively (FIG. 2A, middleand lower curves on the left side of the graph). This positive shift inonset potential upon addition of a Nafion overlayer is also observedwith a Cu substrate. Cu electrodes modified with 2 and 15 μm of Nafionboth showed decreased onset potentials of +0.10 V for Cu electrodemodified with 2 μm of Nafion, and +0.40 V for Cu electrode modified with15 μm of Nafion as compared to −0.19 V for the Cu electrode withoutNafion. This consistent positive shift of onset potential signifies thatthe electrodes with increasingly thick Nafion layers are more efficientat reducing CO₂.

In addition to the positive shift of onset potential, the shapes of theLSVs for the carbon mesh and Cu substrates modified with 15 μm of Nafionare both relatively linear compared to the corresponding LSVs withoutNafion, signifying that the electrochemical behavior of these electrodesare resistive. It is hypothesized herein that this increase inelectrochemical resistance arises from impeded electron transfer throughthe thick Nafion layers. FIGS. 3A and 3B present the electrochemicalimpedance spectroscopy (EIS) data for Nafion-modified carbon and Cuelectrodes, respectively, measured at −0.89 V vs. RHE. Impedance datawas fitted using a Randles circuit (Equation 1), in which R_(f) andR_(ct) refer to solution resistance and charge-transfer resistance,respectively. The equation also contains C_(dl) the double-layercapacitance, and an electrochemical element of diffusion. Z_(W). Whenthe impedance data is fitted using Randles equation, R_(s) and R_(ct)values, as well as the diameter of the semicircle, provides informationregarding the resistivity of the electrocatalyst. Nafion-modified Cu hasa much larger R_(ct) value than bare Cu (Table 1), and R_(ct) increasesas the Nafion overlayer increases, demonstrating increased resistivityof Nafion-modified Cu. This trend correlates well with the LSV from FIG.2B (lowermost curve) because the resistor-like behavior of thickNafion-modified Cu possesses high resistivity. In contrast, the diameterof the semicircle (FIG. 3A, curve through triangle-marked points) and Rndecreases (Table 1) for 15 μm of Nafion on carbon when compared to barecarbon. This decrease in resistivity with increasing Nafion thicknessmay be attributed to the porous nature of carbon, in which the Nafion isembedded within the pores of the carbon rather than acting as anoverlayer.

$\begin{matrix}{R_{s} + \frac{C_{dl}}{\left\lbrack {R_{ct} + Z_{w}} \right\rbrack}} & (1)\end{matrix}$

TABLE 1 Summary of solution resistance (R_(s)) and charge transferresistance (R_(ct)) obtained from electrochemical impedance spectroscopydata fitted to a Randles circuit. Catalyst R_(s) (Ω) R_(ct) (Ω) Carbon144 1961 2 μm Nafion on carbon 67 1978 15 μm Nafion on carbon 384 601 Cu638 402 2 μm Nafion on Cu 646 587 15 μm Nafion on Cu 644 896 PVDF oncarbon 8 25 PVDF on Cu 12 36

Based on the observation that electron transfer is impeded by thickNafion layers, contrasting experiments were performed with a hydrophobicpolymer to block proton transfer to the CO₂ reduction electrodes.Electrodes with a hydrophobic polymer were created by modifying carbonand Cu substrates with a 15 μm thick overlayer of polyvinylidenefluoride (PVDF). FIG. 4A presents the LSVs of carbon and Cu inCO₂-saturated electrolyte. Interestingly, the opposite effect isobserved with PVDF overlayers in which the onset potential is shiftedmore negative in the presence of PVDF. The LSV for an unmodified carbonelectrode possesses an onset potential of −0.29 V (next to lowest curve,at left margin of figure), while the LSV for a PVDF-modified carbonelectrode possesses an onset potential of −0.6 V (lowest curve at leftmargin). Similarly, the LSV for an unmodified Cu electrode exhibits anonset potential of −0.19 V (uppermost line at left margin), while aPVDF-modified Cu electrode exhibits an increased onset potential of−0.55 V (second highest line at left margin). This negative shift inonset potential for PVDF-modified electrodes is attributed to thehindered proton transfer from electrode to CO₂, therefore increasing thedriving force needed to reduce CO₂. This effect is further confirmed byEIS (FIG. 4B) and R_(ct) values. PVDF on carbon and Cu exhibited R_(ct)values of 25Ω and 36Ω, respectively, which signifies that protons areblocked. Furthermore, thick PVDF overlayer on electrodes does notexhibit resistor-like behavior as seen in thick Nafion overlayer onelectrodes, illustrating the differences in the impedance of electrons(Nafion) and protons (PVDF).

Taken together, the data in FIGS. 2A, 2B, 3A. 3B, 4A, and 4B demonstratethat while Nafion overlayers decrease the overpotential of CO₂ reductionfor both carbon and Cu electrodes, PVDF overlayers increase theoverpotential due to their hydrophobicity and blocking of protons.Pursuant to an initial hypothesis, the positive shifts in CO₂ reductionwith Nafion overlayers are simply due to Nernstian changes in the pH atthe polymer-electrode interface (Nafion is a superacid withpK_(a)˜−6).²⁸ To evaluate this hypothesis, CO₂ reduction was conductedunder different pH values. FIG. 5 shows LSVs of a carbon electrode inCO₂-sparged 0.1 M NaHCO₃ buffer at pH 6.8 (lower line at left) and pH2.6 (upper line at left). CO₂ reduction at pH 6.8 has a much earlieronset potential (−0.5 V vs. RHE) and higher current density than CO₂reduction in an acidic medium (−0.9 V vs. RHE). The observation that theincrease in acidity causes the onset potential to shift negative is theopposite of what would be predicted if Nafion's effect on onsetpotential were caused by interfacial pH effects because Nafion isacidic. Instead, Nafion elicits a positive shift in onset potential, andtherefore we conclude that this positive shift is not simply due to pHchanges at the polymer-electrode interface.

Based on the LSV results presented, further investigation was undertakento discern whether CO₂ reduction occurs at the polymer-electrolyteinterface (FIG. 9A), the electrode-polymer interface (FIG. 9B), or theelectrode-electrolyte interface (FIG. 9C). Scanning electron microscopy(SEM) and energy-dispersive X-ray (EDS) analysis were employed to showthe uniformity of Nafion on the electrode. FIGS. 6 -F respectively showthe SEM image (6A), EDS spectrum (6B), and EDS mapping of a 2 μm thicklayer of Nafion on Cu (6C-F). EDS mapping shows that F, O, and S, allelements present in Nafion, are uniformly present on top of the Cuelectrode. The uniform nature of the Nafion overlayer suggests that CO₂reduction is not occurring at the electrode-electrolyte interfaces asmight occur with a nonuniform overlayer (FIG. 9C). FIG. 8 presents arepresentative cross-sectional SEM image of a 2 μm thick layer of Nafionon Cu. EDS mapping of the cross-sectional view clearly shows Cu (FIG.8C) and F (FIG. 8D) from Nafion and also is evidence of the uniformityof the Nafion layer.

CO₂ Reduction on Nafion-Modified Electrodes

To elucidate whether CO₂ reduction is occurring at thepolymer-electrolyte interface or at the electrode-polymer interface. CO₂reduction products were quantified using nuclear magnetic resonance(NMR) spectroscopy (for liquid products) and gas chromatography (GC)(for gaseous products). FIG. 10A summarizes the Faradaic efficiencies(FE) of three detected products (CO, CH₄, and HCOOH) at −0.89 V vs. RHE.Four different substrates (carbon, Cu, Ni, and Zn) were tested toevaluate the effect of the Nafion overlayer. These four substrates weretested without any modification, modified with 15 μm of Nafion, ormodified with 15 μm of PVDF. Unmodified Cu produced CO and HCOOH, whileNafion-modified Cu showed a significantly enhanced CH₄ production of 68%FE, while the Faradaic efficiencies for CO and HCOOH decreased. Thedecrease in CO and concomitant increase in CH₄ strongly suggest that CO₂is reduced by the substrate and is subsequently trapped and stabilizedas a CO intermediate by Nafion, which is then further reduced to CH₄. Cumodified by proton-blocking PVDF only produced trace amounts of CO.Carbon modified with Nafion compared to bare carbon shows an increase inCO and HCOOH, while no CH₄ was made. Similar to PVDF-modified Cu,PVDF-modified carbon produced no carbon-containing products. In responseto the differences in the results between Cu and carbon, two additionalsubstrates were tested. Compared to unmodified Ni, Nafion-modified Nishowed a decrease in CO production and a slight increase in HCOOHproduction. Unmodified Zn produced CO, CH₄, and HCOOH, whileNafion-modified Zn only produced increased formate and CO. Because eachsubstrate yields its own unique set of Faradaic efficiencies for eachproduct, it is concluded that CO₂ reduction occurs at theelectrode-polymer interface with a 15 μm thick Nafion overlayer. Incontrast, if the Faradaic efficiencies for each product were similarregardless of substrate, the conclusion would have been that CO₂reduction occurs at the polymer-electrode interface because the natureof this interface does not depend strongly on the substrate.

In addition to Faradaic efficiencies, product formation can also beexpressed in terms of partial charge density and rates. Partial chargedensities (FIG. 10B) of product formation follow the same general trendsas those of the Faradaic efficiencies, while the rates of productformation (in units of nmol/cm²-s) show disproportionately slower CH₄production rates because CH₄ production requires 8 e⁻/mol as opposed toCO and HCOOH production, which both only require 2 e⁻/mol.

As previously demonstrated by linear sweep voltammetry (FIG. 3 ), thehydrophobic polymer PVDF completely blocks proton transfer in the CO₂reduction reaction. When CO₂ reduction is attempted with an electrodethat is modified by PVDF, no carbon-containing products are made (FIG.10 ). This has been tested on two substrates (a Cu electrode and acarbon electrode), and only very low yields of carbon-containingproducts were made on either substrate modified with PVDF. In otherwords, with PVDF-modified substrates, H₂ is the only product. Thesefindings demonstrate that product selectivity is based on protonavailability and proton transfer rates. Nafion is a highlyproton-conductive polymer that can rapidly shuttle protons, which has asignificant effect on product selectivity. With Nafion, the CO*intermediate can be rapidly protonated, which subsequently leads to CH₄formation.

To further verify that CO₂ reduction is occurring at theelectrode-polymer interface with a 15 μm overlayer, the thickness ofNafion was varied. Faradaic efficiencies as a function of Nafionthickness on a Cu electrode is presented in FIGS. 11A and 11B. Varyingthe thickness of Nafion on a Cu electrode (FIG. 11A) results indifferent Faradaic efficiencies of CO, CH₄, and HCOOH. Without Nafion,Cu produces mostly CO and HCOOH under these experimental conditions.When modified with a 2 μm thick layer of Nafion, CH₄ production isgreatly increased and reaches a maximum Faradaic efficiency of 68.4%when modified with 15 μm overlayer of Nafion. Thicker layers of Nafioncause a decrease in products, which indicates that the hydrogenevolution reaction (HER) occurs on very thick Nafion membranes. Based onthese results, it is posited that CO₂ is reduced at theelectrode-polymer interface to produce CO when thinner membranes areused and that the HER occurs at the polymer-electrolyte interface withthicker membranes.

Partial charge densities (FIG. 11B) for CH₄ follow the same generaltrends as Faradaic efficiencies. However, 2 μm of a Nafion overlayerinhibits HCOOH formation. Rates of product formation (FIG. 11C) showthat on unmodified Cu electrodes, CO and HCOOH formation is similar (4.4nmol/cm² s for CO and 4.3 nmol/cm² s for HCOOH), while no CH₄ isproduced. Cu electrode modified with 2 μm of Nafion exhibits anincreased CO production rate (6.3 nmol/cm² s) and a decreased HCOOHproduction rate (0.8 nmol/cm² s) and still no CH₄ is produced. When theCu electrode is modified with 8 μm thick or 15 μm thick layers ofNafion, CH₄ formation is observed. The rate of CH₄ production is fasterwith a 15 μm Nafion overlayer (6.12 nmol/cm² s) as compared to a 8 μmthick overlayer (6.1 nmol/cm² s). These results signify that Nafionshould be thick enough to trap CO generated from the Cu electrode andthat 15 μm is the optimal Nafion thickness for enhanced CH₄ formation.With thicker Nafion overlayers (30 μm), the CO production rate isslowed, and no CH₄ formation is observed. Furthermore, with extremelythick Nafion overlayers (183 μm), all product rates are significantlydecreased, and no CH₄ is produced. This result suggests that on verythick Nafion overlayers, only the HER is occurring on top of the polymerat the polymer-electrolyte interface.

Voltage-dependent experiments are presented in FIG. 12 . At −0.38 V vs.RHE. CH₄ production reaches 88.0% Faradaic efficiency on a Cu electrodemodified with 15 μm of Nafion. By comparison, previous literaturereports on unmodified Cu electrode at −0.9 V vs. RHE give only 0-1%CH₄.²² These results indicated that a Nafion-modified Cu electrode notonly enhances CH₄ production but also produces it at a significantlydecreased overpotential. Notably, to the best of the inventors'knowledge, the 88% Faradaic efficiency for CH₄ obtained is higher thanany previous literature reports under any experimental conditions. SeeTable 2, herein below. The currently reported best CO₂ to CH₄ catalystshave a Faradaic efficiency of ˜70%.²⁹ In addition to CO, CH₄, and HCOOH,this polymer-modified Cu electrode also produces ethanol and methanol atFaradaic efficiencies of 0.2% and 0.06%, respectively, at −0.98 V vs.RHE. Both partial charge density and rate of CO and HCOOH formationplots (FIGS. 12B, 12C) follow the same general trends as those of theFaradaic efficiencies. However, at −0.38 V, the partial charge densityand rate of CH₄ formation is lower than at −0.9 V due to the lowerdriving force at this decreased overpotential.

Given the remarkably high Faradaic efficiency for CH₄ production byNafion-modified Cu electrodes, several experiments were performed togain insight into the mechanism of CO₂ reduction under these conditions.First was an experiment in which sodium formate was added to thebicarbonate buffer in the absence of dissolved CO₂. This experimentresulted in trace amounts of CO and no CH₄ production, indicating thatCO₂ reduction to CH₄ does not occur via a formate intermediate.Secondly, a CO reduction experiment was performed using a Cu electrodewith a 15 μm thick Nafion overlayer at −0.38 V vs. RHE (the electrodewith the highest Faradaic efficiency for CH₄ production). Thisexperiment yielded 38% Faradaic efficiency of CH₄. The relatively highFaradaic efficiency for CH₄ production using CO-sparged electrolyteindicates that a good portion of the formed CH₄ in the CO₂ reductioncase originates from a CO intermediate. However, the observation thatthe Faradaic efficiency for CO reduction to CH₄ is still significantlylower than the Faradaic efficiency for CO₂ reduction to CH₄ (88%) underthe same experimental conditions suggests that additional factors needto be considered. In the pathway leading to CH₄ formation, theprotonation of CO to CHO on the electrode surface is therate-determining step.¹⁴ Furthermore, previous studies suggest that CH₄formation is pH dependent and that CH₄ formation is favored at lower pHvalues.^(30,31,32,33) CO₂-saturated 0.1 M NaHCO₃ electrolyte has a pH of6.8 while the pH of CO-saturated electrolyte has a pH closer to 9. Theabundance of H⁺ in a more acidic CO₂-saturated electrolyte implies rapidprotonation of the CO intermediate, favoring CH₄ formation. The higherpH of the CO-saturated electrolyte yields less CH₄ due to less H⁺present in the electrolyte.

FIGS. 12A-12C present a proposed mechanism of CO₂ reduction to CO andCH₄. With the addition of 2 H⁺ and 2 e⁻ a CO intermediate is formed withtwo possible resonance structures (FIGS. 12A-12C, dotted box). Each ofthe two structures can either be released as CO or proceed to be furtherreduced to CH₄. FIG. 12 proposes a mechanism to explain the extremelyenhanced CH₄ production on a Nafion-modified electrode at −0.38 V.First, CO₂ is reduced to CO at the polymer-electrode interface. CO thatis not bound to the electrode surface is released as a product, and COthat is bound to the electrode surface (denoted as a ═C═O* intermediate)is stabilized by Nafion, allowing for the subsequent reduction of CO toCH₄ while preventing CO release. CO₂ reduction to CH₄ also may occurthrough an alternative pathway that does not proceed through a COintermediate.

As described herein novel Nafion-modified electrodes have beenfabricated that exhibit significantly enhanced CH₄ production (up to 88%Faradaic efficiency) as a CO₂ reduction product. With variation of thethickness, voltage, and substrate. CO₂ reduction occurs at theelectrode-polymer interface under the conditions that produced enhancedyields of CH₄. It is posited that CO₂ reduction to CH₄ is significantlyenhanced because Nafion helps to stabilize the Cu—CO* intermediate,which allows for the stabilized CO to be protonated and further reducedto CH₄. In addition, the hydrophobic polymer PVDF hinders protontransfer, which results in increased hydrogen production and veryinhibited carbon product formation. Future studies include tuning thehydrophilicity of Nafion to further modulate proton transfer rates byutilizing different polymer overlayer structures.

TABLE 2 Summary of various electrocatalysts for electrochemical CO₂reduction to CH₄ reported in literature. Voltage CH₄ Faradaic ReferencesCatalyst (vs. RHE) Electrolyte Efficiency (%) First Set Cu foil −2.4 VNaClO₄/MeOH 70.5   ¹ Cu-Co electrode −1.19 V 0.1M KHCO₃ 47.5   ² Cu foil−1.2 V 0.1M KHCO₃ 40   ³ Cu electrode −1.04 V 0.1M KHCO₃ 33.3   ⁴ Pdelectrode −0.80 V 0.1M KHCO₃ 2.9   ⁴ Cd electrode −1.23 V 0.1M KHCO₃ 1.3  ⁴ Ni electrode −1.08 V 0.1M KHCO₃ 1.8   ⁴ N-doped graphene −0.86 V 1MKOH 15   ⁵ quantum dots Ti-phthalocyanine −1.58 V — 28.1   ⁶Cu-phthalocyanine −1.23 V — 28.0   ⁶ Cu nanofoam −1.5 V 0.1M KHCO₃ <2  ⁷ Pt GDE −1.32 V 0.5M KHCO₃ 38.8   ⁸ Pt/C 0.0 V — 6.8   ⁹ Cu singlecrystal −1.01 V 0.1M KHCO₃ 6 ¹⁰ Cu mesh −1.2 V 2M KBr 28.8 ¹¹ Cunanocubes (24 nm) −1.1 V 0.1M KHCO₃ 15 ¹² Cu nanocubes (44 nm) −1.1 V0.1M KHCO₃ 22 ¹² Cu nanocubes (63 nm) −1.1 V 0.1M KHCO₃ 10 ¹² Cu foil−1.1 V 0.1M KHCO₃ 18 ¹² Polycrystalline Cu −1.0 V 0.1M KHCO₃ 4.6 ¹³ Cunanoparticles −1.35 V 0.1M NaHCO₃ 76 ¹⁴ Cu₂O/Zn −1.9 V 0.3M KOH in 7.5¹⁵ MeOH Nanoporous carbon −1.6 V 0.1M KHCO₃ 0.18 ¹⁶ Cu electrode −1.05 V0.1M LiHCO₃ 32.2 ¹⁷ Cu electrode −1.05 V 0.1M NaHCO₃ 55.1 ¹⁷ Cuelectrode −0.99 V 0.1M KHCO₃ 32.0 ¹⁷ Cu electrode −0.98 V 0.1M CsHCO₃16.3 ¹⁷ Cu₂O films −1.1 V 0.1M KHCO₃ 5 ¹⁸ Cu₂O films −0.99 V 0.1M KHCO₃<1 ¹⁹ Cu electrode −1.01 V 0.1M KHCO₃ 29.4 ²⁰ Cu electrode −1.04 V 0.1MKCl 11.5 ²⁰ Cu electrode −0.99 V 0.5M KCl 14.5 ²⁰ Cu electrode −1.00 V0.1M KClO₄ 10.2 ²⁰ Cu electrode −1.00 V 0.1M K₂SO₄ 12.3 ²⁰ Cu electrode−0.83 V 0.5M K₂HPO₄ 17.0 ²⁰ Cu electrode −0.77 V 0.1M K₂HPO₄ 6.6 ²⁰ Cuelectrode −0.96 V 0.1M KHCO₃ 22.3 ²¹ Fe electrode −0.98 V 0.1M KHCO₃ 1.1²¹ Ni electrode −1.09 V 0.1M KHCO₃ 1.1 ²¹ Cu electrode −1.0 V 0.1M KHCO₃3 ²² Cu—Ni electrode −1.3 V 0.5M KHCO₃ 20.2 ²³ Ag electrode −1.4 V 0.1MKHCO₃ 0.09 ²⁴ Ni electrode −1.00 V 0.1M KHCO₃ 0.6 ²⁵ Ni electrode −1.08V 0.1M KHCO₃ 1.8 ²⁵ Ni electrode −1.02 V 0.1M KHCO₃ 2.4 ²⁵ Cu sheet−1.00 V 0.1M KHCO₃ 16.3 ²⁶ Cu₂O/carbon black −1.3 V NaCl/MeOH 26.9 ²⁷ Cusheet −1.35 V 0.1M KHCO₃ 44 ²⁸ Cu porphyrin −1.0 V 0.5M KHCO₃ 26 ²⁹ Cuelectrode −1.35 V 0.5M KHCO₃ 5.3 ³⁰ Cu electrode −1.6 V 1.1M KHCO₃ 44 ³¹Cu electrode −2.4 V 0.5M LiClO₄/MeOH 71.8 ³² Cu wire electrode −3.35 VTetraethylammonium 28.1 ³³ perchlorate methanol Cu nanoparticles −1.3 V0.1M KHCO₃ 50 ³⁴ Cu₂Pd −1.2 V 0.1M TBAPF₆/CH₃CN 55 ³⁵ Co protoporphyrin−0.8 V 0.1M HClO₄ 2.5 ³⁶ Polycrystalline Cu −1.4 V 0.5M KHCO₃ 42 ³⁷

Third Set of Experiments

In this third set of experiments, the concepts which were established inthe first two sets of experiments were extended to other substrates. Anumber of experiments were run as described in FIG. 28-38 , to determinethe impact that changing the metal substrate as the electrode forconducting the reduction of CO₂ would have on the Faradaic efficiency ofthe reaction as a function of the use of an overcoating with a change involtage. In experiments which utilized a brass substrate, the brass wasa mixture of 62% Cu, 37% Zn and trace amounts of Fe (<0.15%), Pb(<0.08%) and Sn (<0.005%) by weight. Zinc substrates were also used inthis third set of experiments. The experiments conducted here areanalogous to the copper experiments which are described herein above andwere generally run for a one hour period.

As set forth in FIG. 28 , the Faradaic efficiencies on brass wereimpacted dramatically by the thickness of the Nafion overcoat (FIG. 28 )at −089V vs. RHE over the 1 hour experiment which was conducted, withthe greatest Faradaic efficiency occurring with a Nafion overcoatingthickness ranging from 2-15 μm.

FIG. 29 shows the Faradaic efficiencies of CO and methane gas as afunction of the voltage on brass foil no overcoating (see FIG. 28 above)over the period of the experiment (I hour). The graph presented in FIG.29 evidences that the voltage used for the reduction reaction alsosignificantly impacted the production of methane from CO₂ with a voltagevs. RHE ranging from −0.2 to approximately −2.0 V being effective and avoltage within the range of −1.0 to −1.7 V being particularly effectivefor generating methane gas.

FIG. 30 shows that the Faradaic efficiencies of CO and ethylene gasproduction for the 1 hour experiment conducted at −0.89V vs. RHEproduced using brass electrodes with an overcoating of an admixture ofNafion/PVDF ranging from 0% by weight PVDF to 100% by weight PDVF and athickness of 20-90 μm, showed high Faradaic efficiency for ethyleneproduction at 20-90 μm, by weight PVDF in the Nafion/PVDF admixture. Theinventors note that the PVDF-Nafion overlayer becomes increasingly thickas the weight percentage of the PVDF in the admixture increases. This isan unexpected and commercially relevant result inasmuch as ethylene is aparticularly valuable commercial product.

FIG. 31 shows the Faradaic efficiencies for CO and ethylene productionover a one hour experiment using a brass foil electrode (no overcoating)in acetonitrile/bicarbonate electrolyte solution using a voltage of−0.89V vs. RHE. As evidenced by the data presented in FIG. 31 , theFaradaic efficiency for ethylene gas production was greatest between 50%and 80% by volume of acetonitrile.

FIG. 32 shows that a Nafion coating (15 μm) on a brass electrode usingan acetonitrile/bicarbonate electrolyte solution at −0.89V vs. RHE asindicated dramatically influences the Faradaic efficiency of CO andethylene production and has little impact on methane gas production. Anacetonitrile/bicarbonate solution ranging from 10-60% by volumeacetonitrile provided the highest Faradaic efficiencies in theexperiment.

FIG. 33 shows the impact of acetonitrile on Faradaic efficiency for theproduction of CO, methane, ethylene and formic acid on a zinc foilcoated with 15 μm thick Nafion performed at −0.89 V vs. RHE. As theacetonitrile volume % increased, much more ethylene was produced, littleformic acid was produced at any level of acetonitrile and methane wasmost efficiently produced (high Faradaic efficiency) at approximately10-40 volume % acetonitrile in the electrolyte solution.

FIG. 34 shows the CO Faradaic efficiency as a function of the thicknessof Nafion coating on a zinc substrate at −0.89 V vs. RHE over the onehour period of the experiment. Noted is that the CO Faradaic efficiencyis highest at 2 μm to 15 μm coating thickness and dissipates as thethickness of the coating increases to 90-100 μm.

FIG. 35 shows the Faradaic efficiencies of CO produced from 20-90 μmPVDF and Nafion admixture coating on zinc substrate at −0.89 V vs. RHEover the one hour period of the experiment. Note that the Faradaicefficiency of CO production is highest at low PVDF content and atapproximately 40-60% by weight PVDF. Above 60% PVDF by weight of theadmixture, the Faradaic efficiency is reduced to close to zero.

FIG. 36 shows the Faradaic efficiencies over the 1 hour experiment forCO and ethylene gas produced using a copper substrate overcoated with 52weight % of several different polymers (polyvinylpyrrolidone (PVP),polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyethylenimine(PEI) in admixture with Nafion at −0.89 V vs. RHE. An admixture ofpolyvinyl alcohol and Nafion provided the highest faradaic efficiencyfor the production of ethylene gas among the polymer admixtures tested.

FIG. 37 shows the Faradaic efficiencies for CO and ethylene gas producedusing various polymer blends in Nafion on a copper substrate at −0.89 Vvs. RHE. A represents 100% polytetrafluoroethylene (Teflon). Brepresents 50 weight % each of Teflon and PVDF in admixture. Crepresents 52 weight % Teflon and Nafion in admixture. D represents 40weight % each of Teflon and PVDF in Nafion admixture. E represents 64weight percent Teflon and 30 weight % PVDF in Nafion admixture and Frepresents 30 weight % and 64 weight % PVDF in Nafion. The resultsevidence that a fluoropolymer which excludes Nafion does not produceethylene gas (or even significant concentrations of CO) and theinclusion of a fluoropolymer (Teflon) with Nafion at approximately 50%by weight (52:48) produced a larger concentration of ethylene gas as didpolymer admixtures with greater percentages of fluoropolymer (PVDF) inNafion.

FIG. 38 shows the Faradaic efficiencies for the production of CO,methane, ethylene and formic acid producing using 10 weight %nanoparticulate cuprous oxide (Cu₂O) in admixture with Nafion polymercoated on metal substrates (A and B) or a Cu₂O nanoparticulate coating(a thin film of Cu₂O nanoparticulates without Nafion coated onto metalsubstrates by drop casting from a dispersion of Cu₂O nanoparticulates)on metal substrates (C. D and E) over the one hour experiment at −0.89 Vvs. RHE. A represents the results for the Nafion admixture overcoatingon copper substrate, B represents the results for the Nafion admixtureovercoating on zinc, C represents the results for the Cu₂O thin film onzinc (C), Copper (D) and Nickel (D) substrates. As indicated, theinclusion of Cu₂O in the Nafion overcoating had a significant impact onCO and formic acid production with high Faradaic efficiency for CO.Given the other experiments using fluoropolymers it is anticipated thatthe inclusion of Teflon and/or PVDF in the Nafion polymer (often atweight % great than 50 weight %) is expected to have a substantialimpact in producing methane and ethylene products during CO₂ reduction.

FIG. 39 shows a mechanism for electrocatalysis at a polymer-substrate(catalyst) interface with an embedded cocatalyst in admixture with thepolymer to provide tandem catalysis. Cocatalysts can be smallnanoparticulates (having a diameter ranging from 1 to 500 nm) ornanowires which are dispersed in the polymer overcoating. Alternatively,a molecular species which functions as a cocatalyst may be covalentlyattached to the polymer backbone. By coupling an electrode catalyst thatis selective for a partially reduced intermediate and with amembrane/coating bound catalysis which facilitates further reduction,reference cells can be provided for reducing CO₂ to selectively desiredproducts with high Faradaic efficiencies.

CONCLUSIONS DRAWN FROM THE EXAMPLES

The experiments evidence that the use of a uniform Nafion overcoatingranging from 2 to 15 μm (often 10-15 μm, most often 15 μm) on a copperelectrode at an effective voltage using a bicarbonate solution (with noadditional aprotic solvent in the solution) provides high Faradaicefficiency and dramatically high yield of methane gas.

Also evidenced by the experiments described herein, this workdemonstrates that controlling the hydrophobicity of the electrode andproton availability of the electrolyte strongly dictates the productionof different CO₂ reduction products. Formate production is favored by ahydrophobic electrode, however, too hydrophobic causes mass transportissues because hydrophobic PVDF is less permeable to CO₂. The decreasein proton concentration slows down the protonation of the M-COintermediate to generate CH₄, but promotes M-CO and M-CO couplingchemistry to produce C2+ products. This control of hydrophobicity byusing polymer blends and mixed aprotic-protic solvent systems is afacile and effective method to tune the selectivity of CO₂ reductioncatalysts.

A skilled practitioner can predict carbon-based product selectivity fromCO₂ electrolysis reduction reactions by the design of the electrode, theelectrode's polymer coating (including the thickness of the polymercoating) and the composition of the bicarbonate electrolyte solutionused as the CO₂ source. CH₄ production is favored when electrodes aremodified with a Nafion overlayer and on unmodified electrodes at a verynegative reduction potentials. Formate is favored with hydrophobicelectrodes (e.g. PVDF-Nafion overlayer) and at less negative reductionpotentials. C₂H₄ is favored when Cu alloys are used, when the alloyelectrode is hydrophobic (e.g. PVDF-Nafion overlayer), and when aproticsolvents are used in conjunction with the bicarbonate electrolyte.Further, the inventors have surmised that formate can be furtherenhanced by creating a hydrophobic environment on the electrode and/orin the electrolysis solution. C₂H₄ production can be enhanced byhydrophobic fluoropolymer-Nafion overlayers on an alloy electrode.

In addition, from the description of the present invention, the polymeroverlayers as hosts for tandem catalysis. Cocatalysts can benanoparticles and/or nanowires dispersed in the polymer overlayers ormolecular species covalently attached to the polymer backbone. Bycoupling an electrode-bound catalyst that is selective for a partiallyreduced intermediate and with a membrane-bound catalysis thatfacilitates further reduction, one can envision the ready design ofelectrolysis systems utilizing CO₂ reduction that selectively formdesired products.

Supplemental Information Mass Transport Calculations Effect of MassTransport on CO₂ Electrocatalysis on Nafion/PVDF-Modified Electrodes

The permeability of CO₂ in PVDF and Nafion were taken to be 2.16×10⁻¹⁷mol-cm/cm²-s-Pa and 8.70×10⁻⁶ mol-cm/cm²-s-Pa, two values obtained fromFlaconneche, et al., Oil Gas Sci. Technol.-Rev. IFP. 2001, 56(3) and261-278; Ren, et al., J. Electrochem. Soc., 2015, 162(10), F1221-F1230.The permeability of CO₂ in PVDF-Nafion mixtures were calculated based onthe weight percent of PVDF in Nafion multiplied by the permeability ofCO2 in PVDF added to the weight percent of Nafion multiplied by thepermeability of CO2 in Nafion. The thickness of the PVDF-Nafionoverlayer was determined by cross-sectional SEM. Using the thickness ofthe PVDF-Nafion mixture (18 μm for 4 weight % PVDF in Nafion overlayer)and the pressure of CO2 is 1 atm. the flux of CO₂ through the membraneis calculated to be 4.7×10⁻⁸ mol/cm²-s. This flux value is then comparedto the maximum theoretical rate of consumption of CO₂ at theelectrode-polymer interface. The maximum CO₂ consumption rate isdetermined from the steady state current of the chronoamperometry,assuming all CO₂ is reduced to either CO or HCOOH. Because theseproducts require only 2 e/mol, they consume CO₂ faster than more highlyreduced products such as CH₄. Therefore, assuming a 100% yield of CO orHCOOH is an upper bound for the CO₂ consumption rate. For the Cuelectrode modified with 4 weight percent PVDF in Nafion overlayer, thesteady state current density is −0.21 mA/cm². From this value, the upperbound for the CO₂ consumption rate is 1.1×10⁻⁹ mol/cm²-s, a value lessthan the calculated CO₂ flux. Therefore, these calculations suggest thatCO₂ mass transport is not a limiting factor for this electrode.

However, for the Cu electrodes modified with 56, 60, and 64 weightpercent PVDF in Nafion, the CO2 flux is less than the maximumtheoretical CO₂ consumption. This means at these higher weightpercentages of PVDF in Nafion, CO₂ mass transport does become a limitingfactor and the availability of CO2 at the Cu-polymer interface is anissue.

TABLE S1 Contact angle measurements on PVDF- Nafion-modified Cuelectrodes. Electrode (Weight % PVDF in Nafion) Angle (degrees) Bare Cu28.9 ± 2 0 43.7 ± 2 4  38.0 ± 0.8 8  43.9 ± 0.7 15 49.7 ± 2 18 70.3 ± 130 74.9 ± 3 52 78.7 ± 3 56 84.8 ± 4 60 86.5 ± 2 64 95.1 ± 1 100  124.0 ±0.4

TABLE S2 Mass transport calculations. Electrode Max theoretical CO₂(Weight % PVDF in Nafion) CO₂ flux consumption 4 4.7 × 10⁻⁸ 1.1 × 10⁻⁹ 84.1 × 10⁻⁸ 5.7 × 10⁻⁹ 15 3.0 × 10⁻⁸ 4.5 × 10⁻⁹ 30 1.8 × 10⁻⁸  6.2 ×10⁻¹⁰ 52 8.6 × 10⁻⁹ 1.6 × 10⁻⁹ 56 6.0 × 10⁻⁹ 6.7 × 10⁻⁹ 60 4.5 × 10⁻⁹5.7 × 10⁻⁹ 64 3.5 × 10⁻⁹ 5.7 × 10⁻⁹

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What is claimed is:
 1. A method for CO₂ reduction, comprising: providingan electrode having a layer of a predetermined uniform thickness of apolymeric composition; and placing the electrode with the layer ofpolymeric composition in contact with a solution effective for CO₂reduction, wherein said polymeric composition consists essentially ofNafion polymer or an admixture of Nafion in combination with anotherpolymer and/or a cocatalyst.
 2. The method defined in claim 1 whereinthe polymeric composition is Nafion or Nafion in combination with atleast one additional polymer selected from the group consisting ofpolyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP),polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine(PEI), polytetrafluoroethylene (PTFE) and mixtures thereof.
 3. Themethod defined in claim 1 wherein the polymeric composition includes afluoropolymer.
 4. The method defined in claim 3 wherein thefluoropolymer is polyvinylidene fluoride (PVDF, polytetrafluoroethylene(PTFE) or a mixture thereof.
 5. The method defined in claim 2 whereinthe at least one additional polymer is PDVF.
 6. The method defined inclaim 1 wherein the layer of the polymeric composition is Nafion havinga thickness between approximately 2 μm and approximately 15 μm.
 7. Themethod defined in claim 1 wherein the layer of the polymeric compositionhas a thickness effective to stabilize an intermediate in which CO isbound to the electrode coated with the layer of the polymericcomposition.
 8. The method defined in claim 2 wherein the layer of thepolymeric composition has a thickness between approximately 20 μm andapproximately 90 μm.
 9. The method defined in claim 1, wherein theelectrode is made of a material selected from the group consisting ofcarbon, copper, nickel and zinc and mixtures and alloys thereof.
 10. Themethod defined in claim 1, wherein the electrode is made of a transitionmetal or transition metal alloy.
 11. The method defined in claim 10wherein the electrode is made of copper, zinc, silver, gold, cadmium,nickel, palladium, platinum or an alloy thereof.
 12. The method of claim10 wherein the electrode is made of copper or a copper alloy.
 13. Themethod according to claim 12 wherein the copper alloy is brass (copperand zinc), bronze/phosphor bronze (copper and tin), naval brass (copper,zinc and tin), aluminum bronze (copper and aluminum), berylliumcopper(copper and beryllium), cupronickel (copper and nickel, and optionallyiron and/or manganese), nickel silver (copper with nickel and zinc),copper silver (copper with silver) or copper gold (copper with gold).14. The method defined in claim 13 wherein the copper alloy is brass.15. The method defined in claim 10 wherein the layer of the polymericcomposition has a thickness effective to stabilize an intermediate inwhich CO is bound to the electrode coated with the layer of polymericcomposition.
 16. The method defined in claim 1 wherein the solution is abicarbonate solution or a bicarbonate solution further comprising aneffective amount of an aprotic solvent.
 17. The method according toclaim 16 wherein said aprotic solvent is selected from the groupconsisting of acetonitrile (MeCN), dimethylformamide (DMF),dimethylacetamide DMA), dimethylsulfoxide (DMSO), tetrahydrofuran (THF),propylene carbonate (PC), or an alkyl nitrile (such as propylnitrile,butyl nitrile, adiponitrile, benzonitrile) or a mixture thereof.
 18. Themethod according to claim 16 wherein said aprotic solvent isacetonitrile.
 19. The method defined in claim 1, further comprisingconducting an electrical current through said solution to said electrodeat least in part through the layer of the polymeric composition.
 20. Themethod defined in claim 1 wherein said polymeric composition furthercomprises a cocatalyst.
 21. The method defined in claim 20 wherein saidcocatalyst is in the form of a nanoparticle or a nanowire.
 22. Themethod defined in claim 20 wherein said cocatalyst is made of copper(metallic), cuprous oxide (Cu₂O), cupric oxide (CuO), Zn, zinc oxide(ZnO) or silver (Ag).
 23. The method according to claim 1 wherein saidpolymeric composition is Nafion.
 24. An electrode for CO₂ reduction,comprising: a base or body of electrically conductive material; and alayer of a polymeric composition of a predetermined uniform thicknessranging from 1 μm to 100 μm on a surface of said base or body, whereinsaid polymeric composition consists essentially of Nafion polymer or anadmixture of Nafion in combination with another polymer and/or acocatalyst.
 25. The electrode defined in claim 24 wherein the polymericcomposition is Nafion polymer in the absence of an additional polymer orcocatalyst.
 26. The electrode defined in claim 24 wherein the polymericcomposition further includes polyvinylidene fluoride and mixturesthereof with Nafion polymer.
 27. The electrode defined in claim 24wherein said polymeric composition comprises at least one additionalpolymer selected from the group consisting of polyvinylidene fluoride(PVDF), polyvinylpyrrolidone (PVP), polyethyleneglycol (PEG),polyvinylalcohol (PVA), polyethyleneimine (PEI), polytetrafluoroethylene(PTFE) and mixtures thereof.
 28. The electrode defined in claim 24wherein the polymeric composition includes a fluoropolymer.
 29. Theelectrode defined in claim 28 wherein the fluoropolymer ispolyvinylidene fluoride (PVDF, polytetrafluoroethylene (PTFE) or amixture thereof.
 30. The electrode defined in claim 27 wherein the atleast one additional polymer is PVDF.
 31. The electrode defined in claim24 wherein the layer of the polymeric composition is Nafion having athickness between approximately 2 μm and approximately 15 μm.
 32. Theelectrode defined in claim 24 wherein the layer of the polymericcomposition has a thickness effective to stabilize an intermediate inwhich CO is bound to the electrode coated with the layer of thepolymeric composition.
 33. The electrode defined in claim 24 wherein thelayer of the polymeric composition has a thickness between approximately20 μm and approximately 90 μm.
 34. The electrode defined in claim 24,wherein the electrode is made of a material selected from the groupconsisting of carbon, copper, nickel and zinc and mixtures and alloysthereof.
 35. The electrode defined in claim 24 24-33, wherein theelectrode is made of a transition metal or transition metal alloy. 36.The electrode defined n claims 24 and 35 wherein the electrode is madeof copper, zinc, silver, gold, cadmium, nickel, palladium, platinum oran alloy thereof.
 37. The electrode according to claim 35 wherein theelectrode is made of copper or a copper alloy.
 38. The electrode definedin claim 37 wherein the copper alloy is brass (copper and zinc),bronze/phosphor bronze (copper and tin), naval brass (copper, zinc andtin), aluminum bronze (copper and aluminum), berylliumcopper (copper andberyllium), cupronickel (copper and nickel, and optionally iron and/ormanganese), nickel silver (copper with nickel and zinc), copper silver(copper with silver) or copper gold (copper with gold).
 39. Theelectrode defined in claim 38 wherein the copper alloy is brass.
 40. Theelectrode defined in claim 24 wherein said polymeric composition furthercomprises a cocatalyst.
 41. The electrode defined in claim 40 whereinsaid cocatalyst is in the form of a nanoparticle or a nanowire.
 42. Theelectrode defined in claim 40 wherein said cocatalyst is made of copper(metallic), cuprous oxide (Cu₂O), cupric oxide (CuO), Zn, zinc oxide(ZnO) or silver (Ag).
 43. An electrolysis apparatus comprising: ahousing defining a chamber; at least two electrodes disposed in part insaid chamber and operatively connectable to a voltage source, said twoelectrodes including a working electrode; a first port member or fittingfixed to housing and communicating with said chamber for directing fluidinto said chamber; and a second port member or fitting fixed to housingand communicating with said chamber for conveying fluid out of saidchamber, said working electrode including an electrically conductivebase member and a coating layer of a predetermined thickness of apolymeric composition disposed on said base member, wherein saidpolymeric composition comprises Nafion alone or in combination with anadditional polymer and/or a cocatalyst. 44-61. (canceled)